1. Field of the Invention
Embodiments of the invention relate generally to circuits, devices, and methods adapted for use in systems performing digital signaling. More particularly, embodiments of the invention relate to circuits, devices, and systems capable of selectively performing single-ended and differential signaling, and methods associated with the circuits, devices, and systems.
2. Description of Related Art
Two commonly used methods for transmitting electrical signals via a communications channel (e.g., one or more electrical signal lines) are known as “single-ended signaling” and “differential signaling”. In single-ended signaling, one signal line communicates a variable voltage representing transmitted data and another signal line is connected to a reference voltage, such as ground. In contrast, differential signaling uses two related signal lines to communicate complementary data signals which are combined at a receiver to form the transmitted data (i.e., a difference between the complementary data signal represents the transmitted data).
To detect a signal transmitted by single-ended signaling, a receiving device compares the varying voltage representing the signal with the reference voltage. On the other hand, to detect a signal transmitted by differential signaling, a receiving device compares the respective voltage levels of the complementary signals.
Single-ended signaling and differential signaling are used in a variety of popular communication protocols. For example, single-ended signaling is used in many parallel computer buses, PS/2 mouse and keyboard connectors, the RS-232 communication protocol, etc. Differential signaling is used, for example, in RS-422 and RS-485 communication protocols, PCI-X and USB communication protocols, etc.
Single-ended signaling and differential signaling have certain respective advantages and disadvantages. As a result, some applications prefer the use of single-ended signaling, while other applications prefer the use of differential signaling.
One significant advantage of single-ended signaling over differential signaling is that single-ended signaling generally requires fewer signal connection elements (e.g., signal lines, I/O pads, etc.) than differential signaling. For example, to simultaneously transmit “n” unique data signals, single-ended signaling requires “n” signal lines, plus one additional signal line to transmit the reference voltage. In contrast, to simultaneously transmit “n” unique data signals, differential signaling requires “2n” signal lines arranged in “n” pairs of complementary signal lines. Because single-ended signaling generally requires signal lines, the design and implementation of systems using single-ended signaling tends to be relatively simple and inexpensive, as compared with systems using differential signaling. In addition, because fewer signals are transmitted in single-ended signaling compared with differential signaling, single-ended signaling typically consumes less current than differential signaling.
One significant advantage of differential signaling over single-ended signaling is that differential signaling tends to be more tolerant of noise. The randomly occurring channel noise on constituent signal lines transmitting complementary signals may be cancelled out when the difference between the respective voltage levels of the complementary signals is detected at a receiving device. Additionally, differential signaling typically transmits each pair of complementary signals on a pair of balanced signal lines. As a result, electromagnetic interference tends to induce the same voltage offsets on both complementary signals. Further, receivers in differential signaling systems typically ignore wires' voltages with respect to a fixed reference while single-ended systems rely on the reference voltage. In some cases, the reference voltage of a single-ended system may have different offsets at the respective transmitter and receiver ends of the system due to different impedances between the reference wire and ground at the transmitter and receiver ends.
Some common forms of noise that may impair single-ended signaling systems more than differential signaling systems include crosstalk, simultaneous switching noise (SSN), and inter-symbol interference (ISI).
In general, the term “crosstalk” refers to interference between nearby signal lines due to conductive, inductive, or capacitative coupling. Similarly, the term “simultaneous switching noise” generally refers to undesired electrical effects caused by simultaneous switching of several nearby signal lines. For example, where several nearby signal lines switch from a low voltage level to a high voltage level, the voltage levels of some nearby signal lines may be undesirably raised from low levels to higher levels due to capacitative coupling, or the switching of some signal lines from the high voltage to the low voltage could be delayed due to the capacitative coupling. Alternatively, the simultaneous switching of several signal lines could also affect power and ground systems, temporarily altering their ability to drive the signal lines. Finally, the term “inter symbol interference” generally refers to interference between successive pulses or symbols in a sequentially transmitted signal stream. For example if a transition of a signal is slightly delayed, e.g., due to SSN, a subsequent signal may be detected with an erroneous value.
In view of the foregoing benefits and drawbacks of single-ended and differential signaling, circuit designers and system designers may use single-ended or differential signaling to transmit data in a particular communication channel based on the channel's operating characteristics. For example, to transmit or receive data in a high-speed data channel, differential signaling may be used in order to prevent errors related to noise. On the other hand, to transmit or receive data in a relatively lower speed data channel, or in a device requiring relatively low power consumption, single-ended signaling may be used in order to minimize development and operation costs.